The present invention pertains to marine data acquisition system and more particularly to a method for accurately determining the horizontal magnetic gradient of the earth's field between two known locations.
The value of a method for determining the horizontal magnetic gradient of the earth's field between two known locations is that temporal anomalies could be detected directly from the data set, eliminating the need for a base magnetometer that is normally located onshore some distance from the actual area of interest. The base magnetometer must be monitored by shore personnel and the difference in locations between the area of interest and the base magnetometer can cause problems in itself. This stems from the fact that although the diurnal variation is apporxoimately the same at the same solar time at different points, it differs in detail.
Such a system is offered but past attempts to process the data have shown that the system is influenced by the presence of the ship. Analysis of the data indicates that a bias difference between the two magnetometers exists and varies with heading.
To mathematically compute this bias difference is an impossible task. Any attempt to do so quickly reveals that there are more variables to be solved for than equations. The solution is non-unique.
A gradiometer system can basically be thought of simply as two magnetometers that operate and measure the earth's total intensity magnetic field independently of each other. FIG. 1 illustrates the present configuration of a typical gradiometer system. A magnetometer 12 being towed 1500 feet aft of a marine vessel 14 is referred to as the near magnetometer. A magnetometer 16 being towed 2000 feet aft of marine vessel 14 is referred to as the far magnetometer. Both magnetometers 12 and 16 are sampled simultaneously.
Temporal variations are magnetic anomalies that are functions of time, and over relatively short distances, it can be assumed that their effects are everywhere equal in magnitude and phase at any one moment. Temporal variations can be caused by any number of sources such as radio transmission emanating from the ship, magnetic storms, or diurnal variations that are somewhat cyclic in nature, and constitute the bulk of the temporal variations on most days.
Temporal variations affect both magnetometers equally at the same moment, so any difference in magnitude of the two sensors should be due directly to the horizontal gradient of the earth's field between their locations. By subtracting simultaneous readings measured by the sensors and properly integrating the results, the earth's field minus temporal variations can be computed.
The problem is that due to the near proximity of the ship, its magnetic field (induced and/or due to electric currents in the steel construction) affects both sensors. Because the distances between each sensor and the ship are different, the effects of the ship are not equal relative to each sensor. The result manifests itself as a bias difference between the two sensors. It can be safely assumed that the ship's magnetic field remains relatively constant with constant heading, but problems arise when heading changes reposition the magnetometers relative to the ship's field. The vector of the ship's field changes direction relative to the sensors and the differences between the old and new resultants can change the magnitude and sign of the bias difference.
The earth's magnetic field is a vector quantity with an average magnitude in the neighborhood of 45,000 gammas. General changes in the magnitude and direction of this vector occur slowly over distances of several hundred miles or more. Local anomalies can vary in magnitude by several hundred gammas, which represent a small percentage of this total. From this, it can be assumed that the varying bias difference between sensors is not due to the ship's interaction with the earth's regional field and that local anomalies have little effect. As long as the ship remains on a constant heading, the bias difference will remain relatively constant over distances of many miles. The magnitude of the bias difference cannot be determined for any one moment in time.
FIG. 2 illustrates the bias difference due to the field induced by vessel 14. In this figure it is assumed that vessel 14 is symmetrical and homogeneous. As illustrated, the values of near magnetometer 12 and far magnetometer 16 are approximately equal when vessel 14 is headed due East or due West. When vessel 14 is headed due North, the value indicated by near magnetometer 12 is greater than that of far magnetometer 16 while when vessel 14 is headed due South the value indicated by near magnetometer 12 is less than that of far magnetometer 16. Since marine vessels are never center point symmetrical and homogeneous and a course heading for magnetometer 12 and 16 of exactly due West or due East is rare because of subsea inconsistencies such as currents, wave action, etc., there is always a bias difference between near magnetometer 12 and far magnetometer 16.